1. Field of the Invention
The present invention relates generally to a color image forming apparatus for forming toner images assuming a plurality of colors and, more particularly, to a color image forming apparatus provided with photosensitive drums for the plurality of colors, respectively.
2. Description of the Related Art
In an image forming apparatus such as a copying machine, a printer, a facsimile, etc., an electrophotographic apparatus has been utilized due to a demand for recording on ordinary paper. In such an image forming apparatus, an electrostatic latent image is formed on a photosensitive drum and thereafter developed, thereby forming a toner image. Then, the toner image on the photosensitive drum is, after being transferred onto a sheet, fixed onto the sheet by heating.
In response to a demand for forming color images in recent years, color image forming apparatuses for forming toner images assuming the plurality of colors have been offered. A color image forming apparatus has been proposed which is mounted with the photosensitive drums corresponding to the plurality of colors. According to this method, the images in respective colors can be formed in series, and, therefore, the color images can be formed at a high speed. However, the toners in respective colors on the photosensitive drums are transferred, resulting in an easy-to-occur color deviation on the sheet. A countermeasure against the color deviation has been sought.
FIG. 11 is a view illustrating a construction of a conventional color image forming apparatus. As depicted in FIG. 11, there are disposed in series four photosensitive drums 90-1 through 90-4 corresponding to four colors (M, Y, C, K) respectively as basic colors for color images. An image exposing unit 91 exposes the images in the respective color components on the photosensitive drums 90-1 to 90-4.
The sheet is fed from left to right as seen in the Figure by an electrostatic adsorption belt 93. Accordingly, a first-color toner image on the photosensitive drum 90-1 is transferred onto the sheet by a transferring unit 92-1. Next, a second-color toner image on the photosensitive drum 90-2 is transferred by a transferring unit 92-2. Subsequently, a third-color toner image is transferred by a transferring unit 92-3. Finally, a fourth-color toner image on the photosensitive drum 90-4 is transferred by a transferring unit 92-4.
This type of color image forming apparatus is disclosed in Japanese Patent Laid-Open Publication Nos. 3-149576 and 3-293679. The major problem inherent in this type of color image forming apparatus is, it is pointed out, the color deviation. This color deviation is classified in terms of its causes into (1) a color deviation which occurs at a stage of forming the latent image on the photosensitive drum, and (2) a color deviation which is produced at a stage of transferring the toner image onto the sheet. The former is the color deviation which occurs even in such a case that the sheet is accurately fed, but the latter is the color deviation produced due to an inaccurately fed sheet. According to the known techniques given above, there is disclosed a construction to correct the color deviations derived from expansions and contractions of the units due to fluctuations in temperature as well as from errors in mounting the units.
An eccentricity may be given as a factor of the color deviation peculiar to the apparatus including the four drums. This is the color deviation attributed to differences in eccentric quantity and in eccentric phase between the four photosensitive drums. For this reason, the eccentricity in the exposure position causes a color deviation of the latent image. Further, the eccentricity in the transfer position is a factor in producing an error in the feeding of the sheet.
A mechanism for producing color deviation when exposed due to the eccentricity of the photosensitive drum will be explained with reference to FIGS. 12A and 12B. As illustrated in FIG. 12B, the image exposing unit 91 irradiates a polygon mirror 911 with beams of light from a laser light source 910 via a collimator lens 913, thus effecting a light scan through the polygon mirror 911. Then, the scanning light falls on the photosensitive drum 90 via an f-.theta. lens 912. At this time, if the photosensitive drum 90 has an eccentricity .epsilon., a position of the image surface changes in the direction of an optical-axis. This results in a variation in dot position in a main scan direction. At this time, a variation quantity .DELTA.P1 is given by the following formula (1): EQU .DELTA.P1=.epsilon..times.tan .theta.1 (1)
For example, when .epsilon.=0.15 mm and .theta.1=20 degrees, the dot positional deviation quantity .DELTA.P1 at both edge portions of printing is 54.6 .mu.m. This deviation quantity corresponds to a level great enough to recognize it as a color deviation through human eyes. Hence, this deviation quantity is unacceptable in a color printer targeted on a high image quality.
Further, as illustrated in FIG. 12A, the beam of light has an inclination .theta.2 with respect to a subscan direction. This is intended to prevent the beam of light reflected by the photosensitive drum 90 from traveling back to the laser light source 910. This inclination .theta.2 also induces the dot positional deviation .DELTA.P2 in the subscan direction. The variation quantity .DELTA.P2 at this time is given by the following formula (2): EQU .DELTA.P2=.epsilon..times.tan .theta.2 (2)
Hence, as illustrated in FIG. 13A, it can be appreciated that the eccentricities of the photosensitive drums cause the positional deviations of the latent image both in the main scan direction and in the subscan direction when exposed. In the case of the apparatus including the four drums, since there are scatters in the eccentric quantities and the eccentric phases of the respective photosensitive drums, this color deviation when exposed appears directly as a color deviation on the sheet.
An explanation of the mechanism of producing the color deviation when transferred by transfer unit 92 due to the eccentricities of the photosensitive drums will now be explained. As shown in FIG. 13B, if there exists an eccentricity .epsilon. of the photosensitive drum, and even when the photosensitive drum is rotating at a constant speed .omega., a peripheral speed v of the photosensitive drum in the transfer position is expressed by the following formula (3): EQU v=v0+.epsilon..omega..times.sin .omega.t (3)
Accordingly, the peripheral speed v of the photosensitive drum fluctuates depending on the eccentric quantity .epsilon. of the photosensitive drum. In this case, when the sheet and the endless sheet feeding unit, such as an electrostatic absorption belt or the like, are brought into contact in the transfer position of the photosensitive drum, a force is produced corresponding to the fluctuation in the peripheral speed, which force acts either in a feeding direction or in the reverse direction. It therefore follows that a motion of the electrostatic adsorption belt or the sheet may be disturbed. As a result, color deviation appears on the sheet.